Equis ISSN 2398-2977

Equine infectious anemia virus

Synonym(s): EIA virus, EIAV

Contributor(s): Sarah Binns, Susan Dawson, Melissa Kennedy




  • Family: Retroviridae.
  • Subfamily: Lentivirinae.

Active Forms

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Clinical Effects



  • Reservoirs: horses, donkeys   Donkey  and mules   Mule  .
  • Infectious virus present in blood, milk and semen.


  • Mechanical inoculation of blood by hematogenous insects eg deer and stable flies, or by hypodermic needles.
  • Transmission at mating (infrequent).
  • Vertical transmission via milk (not via placenta).
  • Transplacental transmission can occur, especially if acute infection occurs during pregnancy.

Pathological effects

  • Infected animals develop long-lasting antibody titers.
  • Antigenic variation allows virus to evade host immune response to form relapsing clinical signs.
  • Immune complexes of virus and antibody become deposited on erythrocytes, endothelial cells and renal glomeruli   →   complement activation   →   inflammation and hemolysis.
  • Equine infectious anemia   Equine infectious anemia (EIA)  has a variable presentation.
  • Disease is generally less severe in donkeys than in horses.

Acute EIA

  • Massive viral replication   →   destruction of infected macrophages   →   acute hemolytic disease 7-10 days post-infection   →   widespread hemorrhage and necrosis of tissues.
  • Often fatal.

Subacute EIA

  • Animal recovers from acute symptoms   →   2-4 weeks   →   subsequent hemolytic crisis   →   weakness, edema, petechiation, anemia, icterus and ataxia .
  • Cycle may recur, or disease may be fatal.

Chronic EIA - 'Swamp Fever'

  • Relapsing cycles of pyrexia, weight loss, anemia, and icterus.
  • Thrombocytopenia occurs during viremic episodes; may manifest as petechie, bleeding.
  • Can recur several times, usually within the first year post-infection.
  • Becomes chronic carrier of the virus.
  • Hemagglutining glycoproteins interact with antibodies and complement   →   hemolysis and erythrophagocytosis, plus decreased erythropoeisis and serum iron   →   chronic anemia   Anemia: chronic disease  .

Subclinical EIA

  • Probably follows an unobserved febrile episode.
  • Chronic virus carriage.

Other Host Effects

  • Virus may be carried by asymptomatic horses for many years.


Control via animal

  • Supportive therapy.
  • Isolation of carrier animals.
  • Avoid re-use of needles.
  • Test blood donors.
  • Do not mate infected stallions with seronegative mares.
  • Isolate foals from carrier mares before suckling.

Control via chemotherapies

  • Isoprinosine or ribovirine have been suggested. No experimental data available.

Control via environment

  • Stable horses in fly-proof environment.


  • None available.

Other countermeasures

  • Import restrictions; testing of horses within and from USA.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Burki F, Rossmanith W & Rossmanith E (1992) Equine lentivirus, comparative studies on 4 serologic tests for the diagnosis of equine infectious anemia. Vet Microbiol 33 (1-4), 353-360.
  • Cheevers W P & McGuire T C (1985) Equine infectious anemia virus - immunopathogenesis and persistence. Rev Infect Dis 7, 83 PubMed.

Other sources of information

  • Horserace Betting Levy Board (2016) Codes of Practice. 5th Floor, 21 Bloomsbury Street, London WC1B 3HF, UK. Tel: +44 (0)207 333 0043; Fax: +44 (0)207 333 0041; Email: enquiries@hblb.org.uk; Website: http://codes.hblb.org.uk.